1. Field of the Invention:
The present invention relates to an improvement in a semiconductor laser. The improvement is for obtaining lasers with a low threshold current and stable transverse mode lasing.
2. Description of the Prior Art:
Semiconductor lasers have many advantages such as smallness, high efficiency, direct modulation by its driving current, and etc., and therefore have bright future as light sources for optical communication and optical data processing. Laser for such use necessitates characteristics of stable fundamental transverse mode lasing, low threshold current, high output of light and high reliability.
One example of the conventional semiconductor laser of GaAs/GaAlAs double heterostructure planar type is shown in FIG. 1, which is a sectional elevation view by a plane perpendicular to output light thereof. The conventional laser of FIG. 1 is made by sequential liquid phase epitaxial growth wherein on
a substrate 1 of . . . n-GaAs, which has on its face PA1 selectively diffused regions, 2,2 of . . . p-GaAs (formed by Zn-diffusion), PA1 a first clad layer 3 of . . . n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 4 of . . . (non-doped) GaAs, PA1 a second clad layer 5 of . . . p-Ga.sub.1-x Al.sub.x As, and PA1 a cap layer 6 of . . . p-GaAs.
the following layers are sequentially formed:
Then, on the cap layer 6, an insulation film 7 having a stripe-shaped opening is formed by a photolithographic method. Thereafter a p-side ohmic electrode 8 and an n-side ohmic electrode 9 are formed on the exposed stripe-shaped face of the cap layer 6 and on the bottom face of the substrate 1, respectively.
In the abovementioned type semiconductor laser of FIG. 1, the injected current is effectively confined in a narrow stripe-shaped lasing region by means of the selectively diffused p-type regions 2,2, and also by means of the stripe-pattern p-side ohmic electrode 8. As a result of the narrow confinement of the injected current, its fundamental transverse mode of oscillation becomes stable. However, the structure of FIG. 1 has a problem of using a diffusion step on the substrate 1, and therefore the crystal structure of substrate surface is damaged, and hence the subsequently grown crystal receives undesirable effect therefrom, resulting in short lifetime of the device.